云南白水台泉水地球化学特征及其气候环境指示意义

Geochemical characteristics of spring water in Baishuitai of Yunnan and their indicative significance on climatic environment

为研究岩溶地下水的水文地球化学演化特征,对云南白水台地区雨水和泉水中氢氧稳定同位素(δD、δ^(18)O)组成和微量元素含量进行了为期近5年(2018年1月至2022年10月)以月为频率的连续监测。结果表明:(1)白水台雨水中δD、δ^(18)O具有明显的季节性变化特征,均表现为雨季偏轻,旱季偏重,主要受到水汽来源与蒸发条件的影响;(2)白水台泉水受当地大气降水补给,其δD、δ^(18)O经历了石灰岩基层中不同通道、裂隙网络中的新、老水混合导致的同位素调蓄平滑作用以及与深部来源CO_(2)的氧同位素交换作用,修正性继承了雨水中δD、δ^(18)O的部分特征;(3)泉水中Ca、Mg、Sr、Ba、Si等元素主要来源于喀斯特水体侵蚀下围岩的溶解,其含量变化可能指示了降水量的变化;而Fe、Al、Mn等元素主要来源于大气降水对上覆土壤的淋滤作用,其浓度变化可能反映了降水强度的变化。

Understanding the geochemical characteristics and dynamic changes of groundwater(e.g.,springs,etc.)is an important scientific reference for accurately interpreting the paleoclimatic and environmental information on karst deposits(e.g.,stalagmites,travertine,etc.).In this study,we continuously collected local rainwater samples and S3 spring water in Baishuitai area of Yunnan Province on a monthly frequency for nearly 5 years(January 2018 to October 2022).We analyzed the temporal variations of stable isotopes of hydrogen and oxygen(δD andδ^(18)O)compositions and trace element contents in the two types of water bodies to identify the recharge source and hydrogeochemical evolution process of Baishuitai spring water,and to reveal the source differences of trace elements in spring water and their indicative significance on climate and environment.The results showed as follows.(1)Values of stable isotope composition of rainwater in Baishuitai region of Yunnan had obvious seasonal variation characteristics-high in the dry season and low in the rainy season.This result was mainly affected by water vapor source and evaporation conditions.During the dry season,water vapor mainly came from the stable continental air mass,and the evaporation effect was strong,which resulted in the enrichment of D and 18O in the remaining water vapor mass.During the rainy season,multiple fractionation and condensation processes occurred when water vapor migrated from the sea to the land,due to the influence of the southwest monsoon and the southeast monsoon.D and 18O were severely scoured,thus makingδD andδ^(18)O light in rain during the precipitation process.(2)The stable isotopic compositions of hydrogen and oxygen in Baishuitai spring water were on or near the atmospheric precipitation line,indicating that the spring water was mainly supplied by atmospheric precipitation.During the infiltration of atmospheric precipitation into the zone of shallow circulation runoff,new and old water from different channels and fissure networks was continuously mixed to regulate and store hydrogen and oxygen isotopes in groundwater,resulting a much smaller variation amplitude ofδD andδ^(18)O in spring water than in rainfall.During the rainy season in 2018,2019 and 2022,δD andδ^(18)O in spring water witnessed a similar trend toδD andδ^(18)O in rainwater with a lag of about one month,indicating thatδD andδ^(18)O in water evolved to some extent in underground runoff,so thatδD andδ^(18)O in spring water correctly inherited some of the characteristics ofδD andδ^(18)O in rainwater.TheδD-δ^(18)O scatter points of spring water in Baishuitai region all shifted to the area near the left side of the atmospheric precipitation line.This means thatδ^(18)O in spring water became lighter under the exchange of lighter oxygen isotopes with CO_(2)gas from deep sources,whileδD basically did not change,indicating that atmospheric precipitation was fully mixed with deep source CO_(2)during the infiltration process.The smoothing effect of isotope regulation and storage generated by the mixing of old water,as well as the exchange of oxygen isotope with CO_(2)from deep sources,indicated that the Baishuitai region contains a wide spring area,numerous underground passageways and fissures,and intricate karst features.(3)The trace elements in Baishuitai spring water show two types of dynamic change characteristics.Elements such as Ca,Mg,Sr,Ba,and Si constitute one category,mainly originating from the dissolution of surrounding rocks under karst water erosion.After entering the rainy season,the water-rock interaction was weakened by the dilution of rainfall.Concentrations of these elements decreased to varying degrees in the early rainy season,and increased in the late rainy season to the transition from the rainy season to the rainy and dry season,while these concentrations stayed stable or slightly fluctuated in the dry season.The decrease in precipitation promoted the precipitation of calcium carbonate caused by degassing,which increased the values of Mg/Ca,Sr/Ca and Ba/Ca.At the same time,water within the fractures between soil and bedrock was less disturbed by fresh water inputs.This reduction in the rate of water migration enhanced the interaction between water and rock,resulting in a preferential leaching of elements such as Sr and Ba.The consequent differential changes in the concentrations of these elements suggest that variations in their levels could be indicative of shifts in precipitation patterns.Elements like Fe,Al,and Mn constitute another category,primarily originating from the leaching effects of atmospheric precipitation on the overlying soil.The occurrence of peaks in these elements during the rainy season corresponds to periods of heavy annual precipitation,while peaks observed in the dry season may be associated with the pulse-like action triggered by a single instance of heavy precipitation.